The present invention relates generally to techniques for recovering valuable chemical products. More particularly, the invention relates to highly efficient and economic processes for selectively recovering desired products from liquid mediums in which they are contained along with impurities, involving novel thermally-assisted chromatographic techniques.
As further background, the recovery and purification of acidic compounds such as carboxylic acids and other valuable chemical products from mediums has long been studied in an effort to discover efficient, cost-effective routes for their production. For example, carboxylic acids such as citric acid and lactic acid are manufactured by fermentation in large scale worldwide. Such fermentations provide fermentation broths from which the desired acid must be recovered and purified. Where high volume manufacture is involved, the importance of keeping recovery costs to a minimum cannot be overemphasized.
Recent recovery work has focused on the use of solid polymeric adsorbent materials to recover carboxylic acids from fermentation mediums. In this approach, the fermentation broth is passed over the adsorbent which adsorbs the carboxylic acid, and the carboxylic acid is desorbed in some fashion to provide product. Generally, a wide variety of adsorbents and adsorption/desorption schemes have been proposed.
For example, Kawabata et al., in U.S. Pat. No. 4,323,702, describe a process for recovering carboxylic acids with a material of which the main component is a polymeric adsorbent having a pyridine skeletal structure and a cross-linked structure. The carboxylic acid is adsorbed on the adsorbent, and then desorbed using a polar organic material such as an aliphatic alcohol, ketone or ester.
Kulprathipanja et al., in U.S. Pat. Nos. 4,720,579, 4,851,573, and 4,851,574, teach solid polymeric adsorbents including a neutral, noniogenic, macroreticular, water-insoluble cross-linked styrene-poly(vinyl)benzene, a cross-linked acrylic or styrene resin matrix having attached tertiary amine functional groups or pyridine functional groups, or a cross-linked acrylic or styrene resin matrix having attached aliphatic quaternary amine functional groups. In their work, Kulprathipanja et al. describe xe2x80x9cpulse testsxe2x80x9d conducted under isothermal conditions in which they identify acetone/water, sulfuric acid, and water as desorbents.
South African Patent Application No. 855155, filed Jul. 9, 1985, describes processes in which product acids were recovered from their aqueous solutions. In the adsorption step, the acid-containing solution was passed through a column containing an adsorber resin consisting of a vinylimidazole/methylene-bis-acrylamide polymer, a vinylpyridine/trimethylolpropane trimethacrylate/vinyltrimethylsilane polymer, a vinylimidazole/N-vinyl-Nmethylacetamide/methylene-bis-acrylamide polymer, Amberlite IRA 35 (Rohm and Haasxe2x80x94acrylate/divinylbenzene based polymer containing dimethylamino groups), or Amberlite IRA 93 SP (Rohm and Haas) or Dowex MWA-1 or WGR-2 (Dow Chemical) (these latter three being styrene/divinylbenzene based polymers containing dimethylamino groups). To desorb the acid, water, usually at a temperature of 90xc2x0 C., was allowed to pass through the column. However, the single-pass elution process described involves an inefficient use of heat energy and does not substantially maximize the potential use of the resins to achieve highly concentrated desorbed solutions. Additionally, resins employed in this South African application are relatively thermally unstable and thus substantially degrade during desorption procedures employing hot water.
International Applications PCT/US92/02107 filed Mar. 12, 1992 (published Oct. 1, 1992, WO 92/16534) and PCT/US92/01986 filed Mar. 12, 1992 (published Oct. 1, 1992, WO 92/16490) both by Reilly Industries, Inc., disclose desorbing lactic and citric acid, respectively, from divinylbenzene crosslinked vinylpyridine or other resins using steam or hot water. The resins employed have advantageous adsorption/desorption capacities and are highly thermally stable under the described hot water desorption procedures. Nonetheless, improved processes would provide greater efficiency in the use of the resins and of heat applied to the desorption, and would readily provide desorbed solutions of even higher product concentration.
Still further processes for recovering acid products have been reported. For instance, U.S. Pat. No. 5,412,126 describes processes in which citric acid is adsorbed on a base resin and then stripped using an alkylamine. The free acid is then recovered by dewatering the material and driving the amine off with heat. U.S. Pat. No. 5,032,686 describes a process in which citric acid is separated from sugars using an acid resin, whereas U.S. Pat. No. 5,382,681 describes a process in which an impurity-containing citric acid solution is first treated with base to convert the citric acid to trisodium citrate, whereafter the basic medium is passed over a base resin to separate impurities.
In light of this and other background in the area, there remains a need for improved, effective processes for purifying and recovering carboxylic acids and other valuable products. The present invention addresses these needs.
Accordingly, one preferred embodiment of the invention provides a process for separating an acid from one or more impurities, which involves thermally-managed chromatographic separation and elution phases. Thus, provided is a preferred process for the recovery of an acid from its mixture with an impurity, which includes a chromatographic separation of the acid from the impurity over an adsorbent resin at a first temperature, followed by elution of the acid product from the adsorbent resin at a second temperature higher, e.g. at least 10xc2x0 C. higher, than the first temperature. More preferred processes employ a contacting zone containing an adsorbent which exhibits higher affinity for the acid than the one or more impurities, and increasing affinity for the acid with decreasing temperature. A first solution containing the acid and the impurity is introduced into the contacting zone. In order to establish high operating capacity, the solution can contain the acid at a level which substantially exceeds the capacity of the adsorbent to adsorb the acid. A second solution (eluent) is passed through the contacting zone. This second solution is passed at a first temperature and under conditions which are effective to establish a front of acid separated in the contacting zone from a front of the one or more impurities. The front of acid is then eluted from the contacting zone with a liquid at a second temperature higher, e.g. at least 10xc2x0 C. or 20xc2x0 C. higher, than the. first temperature. In this manner, during the separation phase, the affinity of the adsorbent for the acid is maintained at a relatively high level, which retards the acid and facilitates separation of the acid from the one or more impurities, whereas during the elution phase, the affinity of the adsorbent for the acid is maintained at a relatively low level, resulting in increased amounts of eluted acid over a given period of time. Thus, preferred processes of the invention can utilize traditional adsorption/thermal desorption phenomena in a chromatographic separation to achieve effective purification of an acid product from one or more impurities, and recovery of highly concentrated product fractions.
Particularly preferred inventive processes employ a continuous contacting apparatus including a plurality of resin-filled contacting zones (e.g. resin columns). A chromatographic separation zone is established including a plurality of the contacting zones together containing a sufficient amount of adsorbent to achieve substantial separation of the acid from the impurity. The chromatographic separation zone is operated at a first, relatively low temperature. An elution zone is established, from which the product acid is eluted once substantially separated from the impurity. The elution zone is operated at a second temperature which is higher than the first temperature. After the elution zone, resin-filled contacting zones (now substantially devoid of product) are preferably subjected to a cooling step so as to obtain optimum temperatures for the separation zone in the next cycle in the continuous contacting apparatus. The preferred cooling step includes passing a liquid medium at a temperature lower than the resin through the contacting zone. Continuous contacting apparatuses can be appropriately valved to sequentially subject the contacting zones to separation, heating, elution and cooling steps and carry out thermally-managed chromatographic separation processes of the invention.
In accordance with another aspect of the invention, a thermally-managed chromatographic separation process includes establishing a process wherein a plurality of contacting zones containing adsorbent are sequentially processed, the processing including passing over the adsorbent a liquid medium containing the product at a level exceeding the capacity of the adsorbent for the product, causing the chromatographic separation of the acid product from the impurity at a first temperature, and then eluting the product at a second temperature at least 10xc2x0 C. higher than the first temperature. In this and other aspects of the invention, a portion of a product-containing medium from a prior-processed contacting zone can be included in the eluent used in the chromatographic separation phase.
Another preferred embodiment of the invention provides a process for the dewatering and recovery of an organic acid product from an aqueous solution, which includes chromatographically treating the aqueous solution of acid product over an adsorbent resin using a water-miscible solvent such as an alcohol as a mobile phase, under conditions wherein the acid product is eluted in an alcohol solution substantially free from water. Such a process is advantageously performed by providing a contacting zone containing an adsorbent which exhibits higher affinity for the acid than for water, and introducing an aqueous solution of the acid into the contacting zone. A liquid alcohol is passed through the contacting zone under conditions which are effective to establish a front of the organic acid separated in the contacting zone from a front of the water, and the front of organic acid is eluted from the contacting zone so as to provide an eluate containing the organic acid in solution in the alcohol, wherein the eluate being substantially free from water (i.e. contains no more than about 15% by weight water).
Still another preferred chromatographic process for dewatering an organic acid product includes the steps of:
(a) providing a plurality of contacting zones containing an adsorbent which exhibits higher affinity for the acid product than for water;
(b) sequentially processing the contacting zones through a chromatographic separation zone wherein an alcohol eluent solution and an aqueous solution containing the acid are together passed through the contacting zones to achieve a chromatographic process in which the acid is separated from the water; and
(c) after step (b), sequentially processing the contacting zones through an elution zone so as to elute the acid product from the one or more contacting zones in a solution of the alcohol which is substantially free from water.
Thermally-managed inventive processes provide for the recovery of products in concentrated liquid mediums while efficiently utilizing thermal energy to aid in the recovery, and also while operating in configurations which provide high capacity and reduce adsorbent inventory requirements. Dewatering processes of the invention provide the recovery of acids in an eluate substantially free from water, thus facilitating recovery of the acid and/or facilitating reactions of the acid to be conducted in a subsequent operation. The invention also provides processes which are easy to operate, and which can be employed to recover a wide variety of desired product products in concentrated solutions. Additional preferred embodiments, features and advantages of the invention will be apparent from the following description.